The role of the superior colliculus and structures in the mesodiencephalic junction in the visual guidance of saccadic eye movements has been investigated. Rhesus monkeys are trained and tested on several tasks that permit measurement and control of visual and oculomotor responses to stimuli in the central and peripheral visual field. After unilateral surgical ablations, the monkeys are retested on the visuomotor tasks in order to assess the effect of the brain damage and to study the course of recovery of function. Immediately after the lesions, visual detection abilities are impaired, but this recovers quickly leaving a longer lasting visual-oculomotor neglect. There is also a loss in the accuracy of visually-guided saccadic eye movements characterized by a conjugate horizontal gaze palsy. This effect appears to be dependent upon damage to more rostral structures rather than to the superior colliculus alone. Future experiments will focus upon localization of relevant structures in the posterior-medial thalamus and mesodiencephalic junction that play such a crucial role in visually-guided saccadic eye movements.